The present invention relates to a communication system. More particularly, the present invention relates to a communication system for performing bi-directional communication utilizing asymmetrical communications paths form by a combination of bidirectional communications paths utilizing cables and unidirectional communications paths as typified by satellite communications.
Protocols are utilized for performing data transfer or exchange between different network systems when carrying out communications on an Internet system linking a plurality of different network systems. At higher levels the designs of hierarchical communications protocols are becoming widespread for performing data exchange between application programs.
A routing protocol currently widely used for packet exchange between different network systems is designed under the preconditioned that the same path is used for two-way (or bidirectional communication). Consequently, utilizing as is, one-way (unidirectional) paths such as satellite communications having only unidirectional communications from the broadcast station to the receiver (receiving station) or running applications designed under the precondition of bidirectional communications capability is impossible.
One proposed solution was improving the network layer routing protocol to handle unidirectional paths. However, such a method was not practical since several hundred thousand pieces of communications equipment would have to be modified to accommodate the change. A proxy server has been established to function as a relay of application level protocol by performing asymmetrical routing utilizing unidirectional paths without changing the pre-existing bidirectional routing protocol. The proxy server allowed utilization of asymmetrical communications paths by performing routing that used unidirectional paths.
However, since the various application protocols such as telenet, ftp, http were being standardized and utilized in large numbers according to the application goal, in the abovementioned proxy server method, it became necessary to provide a proxy server to match each individual application protocol. This presented the problem that a large number of proxy servers must be prepared and that newly standardized protocols could not immediately be used since no corresponding proxy protocol was available.
The present invention provides an asymmetrical path communications system utilizing asymmetrical routing with unidirectional paths, without changing the preexisting routing protocol, in network systems built up under the precondition of performing bidirectional packet communications using bidirectional paths.
The communications of the present invention utilizes asymmetrical paths and a network system that is configured under the precondition that packet bidirectional communication is performed using bidirectional paths. The present invention includes a unidirectional path for performing packet unidirectional communication, and a communication device (hereafter called a xe2x80x9creceiver gatewayxe2x80x9d) connected to the network system established at the transmission site of the unidirectional path and connected to a local system separate from the network system. The receiver gateway has a first address substitution element in which the source address of the packet sent from the local system is substituted for an address (hereafter called xe2x80x9cproxy addressxe2x80x9d) assigned beforehand within the network system for the transmission site of the unidirectional path and then sent, after substitution with the substituted address, to the network system, and a second address substitution element in which when the destination address of the packet sent from the unidirectional path is the proxy address, after the proxy address is substituted into the address of the communications device connected to the local system. The packet after substitution is sent to the local system, the receiver gateway has a routing element for transmitting a packet on a unidirectional path when the destination address of the packet send from the network system is the proxy address, and for transmitting the packet on the network when the destination address is not the proxy address.
In the arrangement described above, when performing communications between a communications device (Type 1 communications devices) connected to the local system and a communications device (Type 2 communications device) connected to the network system, the packet sent from the Type 2 communications device to the Type 1 communications device is sent by utilizing a unidirectional path and the packet sent from the Type 1 communications device to the Type 2 communications device is sent by utilizing the network system (bidirectional path). Thus, communication can be performed utilizing asymmetrical paths without changing the preexisting routing protocol on the network system.
In the above mentioned configuration, the first address substitution element for the receiver gateway encapsulates the packet after substitution and sends the encapsulated packet to the transmitter gateway by way of the network system. The routing element for the transmitter gateway is capable of performing routing of a packet extracted from the capsule when a packet sent from the network system is an encapsulated packet. Also, in the above configuration, the receiver gateway further has a communication element for detecting whether the unidirectional path is capable of communication and informing the transmitter gateway of the detection results by way of the network system.
The transmitter gateway has a storage element for storing the detection results communicated by the receiver gateway. The routing element is capable of substituting the destination address of the packet for transmission on the unidirectional path into the proxy address, and after substitution transmitting the packet to the receiver gateway by way of the network system in the event that the unidirectional path cannot be used for communication. The local system can be connected to more than one communications device, and when connected to two or more communications devices, the local system essentially forms a local network system connecting these communications devices.